Vector Network Analyzers (VNA) are used to determine characteristics and parameters of network devices. A linear electrical network may be considered to consist of inter-related circuits or elements that have impedances and perform specific functions. For purposes of discussion, a network may be considered an electrical black box with one or more inputs or outputs. The network may be formed between the test device and a target device that may or may not be connected together by conductors. Measurements of microwave circuits and components involve the characterization of the circuit as a network, and measuring the reflection and transmission coefficients at the network ports.
The behavior of the network may depend on the network constants. Network elements may be passive and contain no energy source or sink besides normal ohmic losses, or be active and contain an energy source or sink. In operation, a signal that sweeps through a range of frequencies is applied to each port of a device under test (DUT). Both forward-transmission measurements and reverse-transmission measurements of a DUT are determined at each frequency in the range.
A simplified block diagram of a typical VNA 100 is illustrated in FIG. 1. VNA 100 includes an RF processing block 110, a signal processing block 120, and signal display block 130. The RF block 110 is coupled to a DUT 115. Signals are applied to the DUT, forward and reverse transmission characteristics are measured, and the characteristics are provided to the signal processing block 120. The signal processing block performs processing on the signals to clear up the signals and otherwise process them for subsequent presentation. The processed signals are then sent to signal display 130 wherein the signals may be displayed on a monitor. The RF block is a vital part of a VNA as the data obtained is used in the processing and display blocks.
In a sampler-based VNA, a signal is applied to a DUT. Data representing resulting transmission signals and reflection signals are then captured with a sampler. An example of a two port sampler-based RF block 200 of a typical VNA of the prior art is illustrated in FIG. 2. RF block 200 includes a signal generator 210, source resistance 211, pulse forming network 212, power splitter 220, samplers 231, 232, 233, and 234, and RF signal inputs 261 and 262. Attached to system 200 is DUT 250. Each of samplers 231–234 includes a strobe port, RF port, and an intermediate frequency (IF) port. As shown, one end of DUT 250 is connected to reference channel A 241 through sampler 231 and test channel A 242 through sampler 232. The other end of DUT 150 is connected to test channel B 243 and reference channel B 244. In operation, the pulse forming network 212 receives a signal from signal generator 210. In typical sampling-based RF blocks the prior art, the pulse forming network is implemented using step recovery diodes to generate pulses. The pulse forming network provides a pulse signal to power splitter 220. Typically, the pulse signal contains harmonics that may range from 0 to 65 GHz. Power splitter 220 splits the received pulse signal into four split pulse signals and distributes the split pulse signals to samplers 231–234. A frequency sweeping RF signal is then applied to one port of the DUT. In the embodiment shown, RF 1 may be applied to the DUT through port 261. RF 2 may be applied to the DUT through port 262. For each RF signal applied, the transmitted and reflected signals are sampled by the VNA samplers to generate intermediate frequency (IF) signals. For example, for an RF signal applied to the DUT from RF port 261, the forward transmission characteristics are sampled by sampler 233 connected to test channel B, and the reflection characteristics are sampled at sampler 232 corresponding to test channel A. The IF signal data for each sampler is then transmitted to the signal processing block from samplers 231–234 for further processing.
The VNA RF block 200 illustrated in FIG. 2 limits the VNA's dynamic range in that in the presence of highly reflective DUTs', RF signal power leakage from one test channel leaks into another channel via the power splitter. In particular, a portion of the RF signal power reflected by the DUT and received by sampler 2 leaks out of that sampler's strobe port and is received by the power splitter. A portion of the power received by the splitter finds its way into the strobe port of sampler 3, and eventually into the RF port of this sampler. This signal power leakage path is illustrated by the dotted line 260 in FIG. 2. Similarly, RF signal power leakage from the RF port of sampler 3 to the RF port of sampler 2 follows the same leakage path illustrated by the dotted line 260 in FIG. 2.
Typical isolators and amplifiers can not be used to reduce, isolate or prevent the forward and reverse transmission signals from propagating back through the sampler and power splitter from one channel to another channel because they are currently not available over the band of 0 to 65 GHz or greater. Typically, signal power leakage can not be calibrated out, and limits the dynamic range of the VNA and its ability to characterize accurately highly reflective device properties such as the stop bands of filters and diplexers.
What is needed is an improved RF reverse isolation block in a sampler based-VNA that overcomes the limitations of the prior art by increasing the isolation between VNA channels, and thus the VNA's dynamic range.